Up to now, it has been strongly desired to develop a high performance of light material provided with high strength and high elasticity in widespread technical fields including automobiles, aircraft as well as the other fields of industries, thus, from this point of view, carbonaceous fibers or molded carbonaceous materials to be used for composite material have attracted a public attention. In particular, the process for producing a carbon fiber from a carbonaceous pitch has been regarded as important as a process for producing a high performance carbon fiber at a low cost.
By conventional technology, however, it is quite difficult to obtain the long filament of carbon fiber needed for high performance of products, due to the fragility of the pitch fiber because of its small tensile strength as low as 0.01 GPa.
According to the process disclosed in Japanese Patent Publication No. 12740/76 a long filament carbon fiber can be produced from a pitch fiber, by dropping and accumulating a spun yarn into a wire net basket, infusibilizing the spun yarn in the wire net basket, furthermore conducting a primary heat treatment at a temperature of 700.degree. C. or more, thus making the tensile strength of the yarn to be 0.2 GPa or more, in addition, pulling the yarn upwards from said basket to wind it out or while winding it, carbonizing it at a temperature of about 1,500.degree. C., thus obtaining a carbon fiber. This process, however, has a tendency to generating kinks or twists, and is liable to cause curvature of the yarn when the yarn is accumulated, which results in generating a notable irregularity of the yarn surface of the finally produced carbon fiber, and causes the yarn to be of poor appearance, in addition, it results in notably reducing the strength of the area of curvature, and causes the yarn to break frequently, thus it becomes difficult to obtain a high quality of final products. Such advantages have been impossible to be substantially improved.
In addition, a process as disclosed in U.S. Pat. No. 4,138,525 yields a carbon yarn by carbonizing after the treatment wherein by melt spinning mesophase pitch, and once winding the spun yarn on a bobbin, putting a part of the spun yarn on a wire net dish, oxidizing it under an oxidizing atmosphere at a temperature of 250.degree. to 500.degree. C. so as to increase the strength of the yarn, thus making the yarn easy to be processed. However, this method processes in an oxidizing atmosphere at a temperature range of 400.degree. to 500.degree. C., that is, oxidization is conducted at an excessively high temperature, which results in reducing the final yarn strength of the carbon fiber in the final products. Furthermore, it requires that first the yarn be once wound up, next oxidizing a part of the yarn while pulling it upwards, which results in reducing the efficiency of the production.
A process disclosed in Japanese Patent Application Laid-open Nos. 81320/85 and 21911/85 conducts a primary heat treatment (preliminary carbonizing) under a non-oxidizing atmosphere below a given temperature, after infusibilizing the wound bobbin as it is. However, these methods cause insufficient gas permeability during infusibilization or preliminary carbonization when the winding thickness of the pitch fiber on the bobbin becomes thicker. This causes fusion and sticking among the filaments, making it difficult to rewind the wound yarn on the bobbin after the primary carbonization, and results in making it liable to form a fluff of carbon fiber around the yarn on rewinding, thus remarkably reducing the merits of thus obtained carbon fiber or graphite fiber as commercial goods.
Furthermore, insufficiency of gas permeability would increase the irregularity of the degree of infusibilization, and increase the inhomogeneity of the strength of the final products of carbon fiber or graphite fiber.
These disadvantages were largely overcome by a method of utilizing the gas permeable bobbin as disclosed in Japanese Patent Application Laid-open No. 173121/85, the efficiency of manufacturing in this method, however, is not still so satisfactory, and a further improvement has been needed.
A process disclosed in Japanese Patent Application Laid-open No. 128020/80 obtains carbon fiber by melt spinning the yarn, by drawing it with a godet roller, and passing it continuously through the air-heating thermosetting furnace for infusibilization at a yarn rate of 0.15 m/min, subsequently by passing it continuously also through a carbonizing furnace. Although this method can infusibilize the yarn homogeneously so as to reduce the irregularity of its properties thus obtaining a good appearance of carbon fiber, it has a disadvantage in making the operation difficult to continue due to breakage of fiber bundle during its infusibilization, since the spinning treatment oil (finish) which is added to the fiber bundle is decomposed while the infusibilization temperature increases.
On the other hand, methods such as the one disclosed in Japanese Patent Publication No. 42696/73 have been known, wherein an air mixture containing 0.1 to 10% NO.sub.2 is used as an atmospheric gas at infusibilization, and the other disclosed in Japanese Patent Application Laid-open 75828/74 wherein a mixed gas of chlorine and oxygen is used to enhance an infusibilization speed.
Although these methods have an advantage in increasing the infusibilization speed, they have disadvantages not only in generation of breakage of fiber bundles at infusibilization by passing them through linearly and continuously as thread line, but also in that explosion or burning is liable to occur since the reaction rushes in a treatment under a high temperature, in addition, the treatment of strong oxidizing gas makes the apparatus liable to corrode, which results in a short life of the apparatus.
As stated above, it has been desired to find a method which is free from breakage of fiber bundles due to disturbance of sizing performance of treatment oil, which is possible to a rapid infusibilization so as to increase the amount of production per hour, and which can obtain a high quality of the finally produced pitch base carbon fiber in long filaments without inhomogeneity of their strength, in high strength, in high elasticity, in a good appearance, and in a very small amount of fluff in treatment.